1. Field of the Invention
This invention relates generally to an axle for motor vehicles in which power is transmitted from a driveshaft to axle shafts. More particularly, the invention pertains to a system for circulating and cooling hydraulic lubricant in such an axle.
2. Description of the Prior Art
Hydraulic lubricant held in a reservoir at the bottom of an axle assembly housing near the ring gear of an axle differential is heated due to friction produced at bearings in the housing and by energy transferred to the lubricant from the ring gear as it rotates in the housing through the lubricant reservoir. In addition, the lubricant is heated by radiation from vehicle exhaust systems and high ambient air temperatures.
Axle lubricants are adversely affected by high temperature. Their performance deteriorates gradually when their temperature is excessive. Lubricant deterioration and excessive temperatures can cause the bearing, seals, gears and lubricant to fail causing axle failure and increased costs.
Various techniques have been developed for transferring heat from an axle lubricant so that the operating temperature of the lubricant is maintained at an acceptable temperature. For example, to cool axle lubricant operating in a high torque and high temperature environment, lubricant from the axle reservoir is circulated by an hydraulic pump through an external cooler wherein heat is exchanged to ambient air and the lubricant is returned through a closed hydraulic system to the reservoir in the axle housing. Other prior art describes use of heat pipes to cool lubricant in internal combustion engines and axle housings.
It is known to use an axle housing formed with fins used to increase its surface area and to enhance transfer of heat from the lubricant, which circulates through bearings that support the differential mechanism, to outside air. Pumping action of the differential ring gear circulates lubricant to the heat exchanger, which is formed intrinsically with the axle housing.
Each of the techniques in the prior art for exchanging heat from an hydraulic lubricant to the ambient requires enhanced heat transfer capability, usually in the form of a finned heat exchanger or use of an hydraulic pump, in order to increase the surface area through which heat is exchanged from lubricant to air or to circulate the lubricant at a sufficient flow rate so that an acceptable level of heat transfer occurs. A pump and finned heat transfer surfaces add cost to the system, and the pump adds to the power load. Preferably, an axle lubricant system would include means for transferring a sufficient amount of heat to ambient air without need for an external power source to drive a lubricant pump and without need to enhance heat exchange capabilities of a conventional axle housing.